1. Field of the Invention
This invention relates to filled compositions of perfluoropolymer comprising a low melting fluoropolymer additive.
2. Description of Related Art
Plenum cable is cable used for data and voice transmission that is installed in building plenums, i.e., the spaces above dropped ceilings or below raised floors that are used to return air to conditioning equipment. The cable comprises a core which performs the transmission function and a jacket over the core. Typical core constructions include a plurality of twisted pairs of insulated wires or coaxially-positioned insulated conductors.
Cable jackets of polyvinyl chloride (PVC) and flame retardant additives are known for plenum cable, but the resultant compositions do not pass the National Fire Protection Association (NFPA)-255 burn test (Surface Burning of Building Materials), which requires non-flammability and low-to-no smoke emission. UL 2424, Appendix A, provides that cables tested in accordance with NFPA-255 must have a smoke developed index (hereinafter Smoke Index) of no greater than 50 and a flame spread index (Flame Spread Index) of no greater than 25. Heretofore, these attributes of plenum cable jackets have been evaluated by UL-910 (NFPA-262—Standard Method of Test for Flame Travel and Smoke of Wires and cables for Use in Air-Handling Spaces), but as concerns about fire safety have risen, it has been found that cable jackets of PVC composition that pass the NFPA-262 test do not pass the more severe NFPA-255 test.
Cable jackets of tetrafluoroethylene/hexafluoropropylene (FEP) copolymer are also known for plenum cable, that do pass the NFPA-255 burn test. Such FEP has a melt flow rate (MFR) of 2-7 g/10 min, which means that it has a high melt viscosity. Because of this high melt viscosity, this FEP has the disadvantage of high production cost cable jacket, because this FEP is only capable of being extruded at a rate (line speed) of up to about 120 ft/min. Higher MFR (lower melt viscosity) FEP has been tried as cable jacket, but such jacket does not pass the NFPA-255 test. As the MFR increases above 7 g/10 min, the resultant lower melt viscosity of the FEP causes it to drip and smoke, resulting in a Smoke Index of greater than 50. It is noteworthy that this FEP is not flammable, i.e. it simply melts and drips and does not form a carbonaceous char. The same is true with other high MFR melt-fabricable perfluoropolymers.